However, adding wireless connectivity to a product is no easy task. Before even beginning the design phase, designers need to choose a wireless protocol, which can be daunting. For example, some wireless standards operate in the popular unlicensed 2.4 GHz spectrum. Each of these standards represents a trade-off in range, throughput, and power consumption. Choosing the best protocol for a given application requires careful evaluation of its requirements based on its characteristics.
Then, even with highly integrated modern transceivers, designing radio frequency (RF) circuits remains a challenge for many design teams, leading to cost and schedule overruns. Furthermore, RF products require operational certification, which can be a complex and time-consuming process.
One solution is to base the design on a certified module using a multi-protocol system-on-a-chip (SoC). This eliminates the complexity of RF designs employing discrete components and allows for flexible selection of wireless protocols. This modular approach provides designers with a plug-and-play wireless solution, making it easier to integrate wireless connectivity into products and obtain certification.
This article explores the advantages of wireless connectivity, examines some key benefits of 2.4 GHz wireless protocols, briefly analyzes hardware design issues, and introduces suitable RF modules from Würth Electronics. It also discusses the certification process required to meet global regulations, considers application software development, and introduces a software development kit (SDK) to help designers get started using the module.
Advantages of multi-protocol transceivers
No single short-range wireless domain dominates, as each sector makes trade-offs to meet its target applications. For example, greater range and/or throughput come at the cost of increased power consumption. Other important factors to consider include interference immunity, mesh networking capabilities, and Internet Protocol (IP) interoperability.
Among the various established short-range wireless technologies, three stand out as clear leaders: Bluetooth Low Energy (Bluetooth LE), Zigbee, and Thread. They share some similarities due to the common DNA within the IEEE 802.15.4 specification, which describes the physical (PHY) and media access control (MAC) layers of low-data-rate wireless personal area networks (WPANs). These technologies typically operate at a frequency of 2.4 GHz, although Zigbee has some sub-GHz variants.
Bluetooth LE is suitable for IoT applications, such as smart home sensors, where data transfer rates are moderate and infrequent (Figure 1). The interoperability of Bluetooth LE with Bluetooth chips hosted in most smartphones is also a significant advantage for consumer-facing applications such as wearable devices. The main drawback of this technology is the need for an expensive and power-intensive gateway to connect to the cloud and bulky mesh networking capabilities.
Zigbee is also a good choice for low-power and low-throughput applications in industrial automation, commercial, and home environments. Its throughput is lower than Bluetooth LE, while its range and power consumption are similar. Zigbee does not interoperate with smartphones and does not offer native IP functionality. A key advantage of Zigbee is that it was designed from the ground up for mesh networking.
Like Zigbee, Thread operates using IEEE 802.15.4 PHY and MAC and is designed to support large mesh networks of up to 250 devices. Thread differs from Zigbee in that it uses 6LoWPAN (a combination of IPv6 and low-power WPAN), making connectivity with other devices and the cloud straightforward, albeit through network edge devices called border routers. (See “A Brief Guide to Important Things in Short-Range Wireless Technologies”.)
While standards-based protocols dominate, proprietary 2.4 GHz protocols still exist in niche markets. Although they limit connectivity with other devices equipped with chips from the same manufacturer, such protocols can be fine-tuned to optimize power consumption, range, interference immunity, or other critical operating parameters. The IEEE 802.15.4 PHY and MAC are fully capable of supporting proprietary 2.4 GHz wireless technologies.
The widespread adoption of these three short-range protocols, along with the flexibility offered by proprietary 2.4 GHz technology, has made it difficult to choose the right protocol for the broadest range of applications. Previously, designers had to choose one wireless technology and then redesign the product if a variant of the different protocol was needed. However, because these protocols use PHYs based on similar architectures and operate in the 2.4 GHz spectrum, many chip vendors offer multi-protocol transceivers.
These chips allow a single hardware design to be reconfigured for multiple protocols by uploading new software. Even better, the product can come with multiple software stacks, with switching between each stack monitored by a microcontroller unit (MCU). For example, this could allow a smart home thermostat to be configured from a smartphone using Bluetooth LE before the device switches protocols to join a Thread network.
Nordic Semiconductor's nRF52840 SoC supports Bluetooth LE, Bluetooth Mesh, Thread, Zigbee, IEEE 802.15.4.ANT+, and a proprietary 2.4 GHz stack. The Nordic SoC also integrates an Arm® Cortex-M4® MCU (responsible for RF protocols and application software) along with 1 megabyte (MB) of Flash memory and 256 kilobytes (KB) of RAM. In Bluetooth LE mode, the SoC delivers a maximum raw data throughput of 2 megabits per second (Mbits/s). At 3 dB, its transmit current consumption is 3.0 mA with a 5V DC input supply, and with a reference output power of 1 milliwatt (dBm), the receive (RX) current consumption is 6.4 mA at a raw data rate of 1 Mbit/s. The nRF52840 has a maximum transmit power of +8 dBm and a sensitivity of -96 dBm (1 Mbit/s for Bluetooth LE).
The Importance of Good RF Design
While Nordic's nRF52840 and other wireless SoCs are very powerful devices, they still require considerable design skill to maximize their RF performance. In particular, engineers need to consider factors such as power supply filtering, external crystal timing circuitry, antenna design and placement, and, crucially, impedance matching.
The key parameter that distinguishes a good RF circuit from a bad one is its impedance (Z). At high frequencies, such as 2.4 GHz used in short-range radio, the impedance at a given point on an RF trace is related to the characteristic impedance of that trace, which in turn depends on the printed circuit board substrate, trace size, distance from the load, and the impedance of the load.
It has been proven that when the load impedance (antenna for the transmitting system, transceiver SoC for the receiving system) equals the characteristic impedance, the measured impedance remains constant at any distance from the load trace. Therefore, line losses are minimized, and maximum power is transferred from the transmitter to the antenna, improving robustness and range. Thus, building a matching network to ensure that the impedance of the RF device equals the characteristic impedance of the printed circuit board trace has become a good design practice. (See "Bluetooth Low Power SoCs and Tools Compatible with Bluetooth 4.1, 4.2, and 5 to Address IoT Challenges (Part 2)").
Matching networks consist of one or more parallel inductors and series capacitors. The challenge for designers is selecting the optimal network topology and component values. Manufacturers often provide simulation software to aid in matching circuit design, but even when good design principles are followed, the final circuit frequently exhibits disappointing RF performance, lacking range and reliability. This leads to further design iterations to modify the matching network (Figure 2).
Figure 2: The Nordic nRF52840 requires external circuitry to utilize its functionality. This external circuitry includes input voltage filtering, support for external crystal timing, and connection to the SoC's antenna (ANT) pin, as well as impedance matching circuitry between the SoC and the antenna. (Image source: Nordic Semiconductor)
Advantages of modules
Using discrete components to design short-range wireless circuits has some advantages, particularly in reducing bill of materials (BoM) costs and saving space. However, even if designers follow one of the many excellent reference designs offered by the SoC vendor, other factors such as component quality and tolerances, board layout and substrate characteristics, and end-device packaging can significantly affect RF performance.
Another approach is to establish wireless connectivity around third-party modules. These modules are fully assembled, optimized, and tested solutions that enable "plug-in" wireless connectivity. In most cases, these modules are already certified for the global market, saving designers the time and money required for RF regulation certification.
Using modules has some drawbacks. These include increased cost (depending on quantity), larger final product size, reliance on a single vendor and their ability to ship in bulk, and (sometimes) a reduced number of accessible pins relative to the SoC on which the module is based. However, if the simplicity of the design and faster time to market outweigh these drawbacks, then modules are the answer.
One example of a module based on the Nordic nRF52840 is Würth Electronics' Setebos-I 2.4 GHz radio module 2611011024020. This compact module measures 12 × 8 × 2 mm, features a built-in antenna, a cover to minimize electromagnetic interference (EMI), and comes with firmware supporting Bluetooth 5.1 and the proprietary 2.4 GHz protocol (Figure 3). As mentioned above, the module's core SoC also supports Thread and Zigbee, with appropriate firmware added.
Figure 3: The Setebos-I 2.4 GHz radio module has a compact form factor, an integrated antenna, and a cover for limiting EMI. (Image source: Würth Electronics)
The module accepts an input of 1.8 to 3.6 V and consumes only 0.4 microamps (μA) in sleep mode. It operates in the Industrial, Scientific, and Medical (ISM) band, centered at 2.44 GHz (2.402 to 2.480 GHz). Under ideal conditions, it delivers 0 dBm of output power, a site-to-site line range of up to 600 meters (m) between transmitter and receiver, and a maximum Bluetooth LE throughput of 2 Mbits/s. The module has a built-in quarter-wavelength (3.13 cm) antenna, but the range can be extended by connecting an external antenna to the aforementioned ANT terminal on the module (Figure 4).
Figure 4: The Setebos-I 2.4 GHz radio module includes an external antenna (ANT) pin for extending radio range. (Image credit: Würth Electronics)
The Setebos-I radio module accesses the nRF52840 SoC's pins via pads. Table 1 lists the function of each module pin. Pins "B2" through "B6" are programmable GPIOs that can be used to connect sensors such as temperature, humidity, and air quality devices.
Table 1 shows the pin names of the Setebos-I 2.4 GHz radio module. LED outputs are used to indicate radio transmission and reception. (Image source: Würth Electronics)
Short-range wireless product certification
Although the 2.4 GHz band is an unlicensed spectrum allocation, wireless devices operating in this band are still required to comply with local regulations, such as those set by the U.S. Federal Communications Commission (FCC), the European Declaration of Conformity (CE), or the Japan Telecommunication Engineering Centre (TELEC). Compliance with regulations requires product testing and certification, which can be time-consuming and expensive. If any part of the RF product fails testing, a completely new submission is necessary. If the module is to be used in Bluetooth mode, a Bluetooth Special Interest Group (SIG) Bluetooth list is also required.
Module certification does not automatically grant certification to the end product that uses the module. However, it typically reduces the certification of the end product to paperwork rather than an extensive retesting task—provided they do not use other wireless devices such as Wi-Fi. The same is usually true when obtaining a Bluetooth list. Once certified, products using the module will bear a label indicating FCC, CE, and other relevant ID numbers (Figure 5).
Figure 5: Example of an ID label attached to a Setebos-I module to show that it has passed CE and FCC RF certifications. End products can typically inherit these certifications without requiring retesting through a few simple paperwork tasks. (Image credit: Würth Electronics)
Module manufacturers typically obtain RF certification (and Bluetooth listings, if applicable) for their modules in the regions where they intend to sell their products. Würth Electronics has done this for its Setebos-I radio module, although it must be used with factory firmware. In the case of Bluetooth operation, the module is pre-certified, provided it is used with Nordic's S140 Bluetooth LE factory stack or the stack provided via the company's nRF Connect SDK software development kit.
Würth and Nordic firmware are rugged and suitable for any application. However, if designers decide to use open standard Bluetooth LE or a proprietary 2.4 GHz stack, or a reprogrammable module from another commercial vendor's stack, a certification program from scratch is required for the intended operating area.
Development tools for Setebos-I radio modules
For advanced developers, Nordic's nRF Connect SDK provides comprehensive design tools for building application software for the nRF52840 SoC. The nRF Connect extension for VS Code is the recommended integrated development environment (IDE) where the nRF Connect SDK can be run. Alternative Bluetooth LE or 2.4 GHz proprietary protocols can also be uploaded to the nRF52840 using the nRF Connect SDK. (See the comments above for the impact on module certification.)
The nRF Connect SDK works with the nRF52840 DK development kit (Figure 6). This hardware uses the nRF52840 SoC and supports prototyping and testing. Once the application software is ready, the nRF52840 DK can act as a J-LINK programmer, porting code to the nRF52840 flash memory of the Setebos-I radio module via the module's "SWDCLK" and "SWDIO" pins.
Figure 6: Nordic's nRF52840 DK can be used to develop and test application software. This development kit can then be used to program other nRF52840 SoCs, such as the SoC used on the Setebos-I module. (Image credit: Nordic Semiconductor)
Application software built using Nordic development tools is intended to run on the nRF52840 embedded Arm Cortex-M4 MCU. However, the final product may already be equipped with another MCU that developers want to use to run application code and monitor wireless connectivity. Alternatively, developers may be more familiar with development tools for other commonly used host microprocessors, such as STMicroelectronics' STM32F429ZIY6TR, which is also based on the Arm Cortex-M4 core.
To enable external host microprocessors to run application software and monitor the nRF52840 SoC, Würth Electronics provides a Wireless Connectivity SDK. The SDK is a suite of software tools that enables rapid software integration of the company's wireless modules with many popular processors, including the STM32F429ZIY6TR chip. The SDK consists of C language drivers and examples that communicate with connected radio devices using the underlying platform's UART, SPI, or USB peripherals (Figure 7). Developers only need to port the SDK C code to the host processor. This significantly reduces the time required to design software interfaces for the radio modules.
Figure 7: The wireless connectivity SDK driver enables developers to easily drive the Setebos-I radio module via the UART port using an external host microprocessor. (Image source: Würth Electronics)
The Setebos-I radio module uses a command interface for configuration and operation. This interface provides up to 30 commands to perform tasks such as updating various device settings, sending and receiving data, and placing the module into one of several low-power modes. Connected radio devices must be running in command mode to use the wireless connectivity SDK.
in conclusion
Determining a single wireless protocol for connected products can be tricky, and designing radio circuitry from scratch is even more challenging. Würth Electronics' Setebos-I and other radio modules offer not only flexibility in protocol selection but also plug-and-play connectivity solutions that meet the regulatory requirements of various operating regions. The Sebetos-1 module comes with Würth's wireless connectivity SDK, enabling developers to easily and quickly control the module using their own host MCU of choice.
